After four or six normal cleaning in this fashion--or should you encounter a barrel which copper fouls badly, it is a good idea to use a good copper solvent. Sweets 7.62 is the best we have found.
Sweets 7.62 should be used on a patch or a cotton mop as it will "eat a brush alive". We suspect that a bore mop has the ability to "carry" more of the solvent into the bore and may be better suited for this operation. they can also be cleaned out with hot water, dried and re-used.
Apply the Sweets 7.62 solvent to the bore, swabbing back and forth as you do, let it set and work for 10-15 minutes and then push a patch through the bore, If the patch comes out with a green color jut repeat the operation until patches come out clean. DO NOT LEAVE SWEETS IN THE BORE FOR MORE THAN 20 MINUTES AT A TIME!
Quicksilver and Sweets 7.62 do not contain rust preventatives, al always follow up with a solvent that does, like Shooter's Choice.